In the internal combustion engines of today as well as the mechanisms associated therewith such as automatic transmissions, a substantial amount of copper is employed in the construction thereof. However, some of the most commonly used additives in lubricating oil compositions servicing the internal combustion systems, e.g., gear oils (automatic transmission fluid) contain compounds which are highly corrosive to copper. Specifically, among the more effective agents which have been developed for compounding with lubricants to improve extreme pressure and wear properties are sulfur containing organic compounds, for example, sulfurized triisobutylene, sulfurized diisobutylene, sulfurized terpene, sulfurized hydrocarbon oils, vegetable oils, animal oils, xanthate esters, organic polysulfides, particularly polyalkyl polysulfides which contain active sulfur or sulfur compounds which are corrosive to copper. In addition, those hydrocarbon oils derived from high sulfur containing crude oils wherein the sulfurous compounds are not thoroughly removed in refining are often corrosive to copper elements in automotive systems.
To solve this problem of copper corrosion, the prior art has employed various copper corrosion inhibitors with a varying degree of effectiveness. One class of such inhibitors are disclosed in U.S. Pat. Nos. 2,719,125 and 2,719,126 which are directed to copper corrosion inhibited lubricating oil compositions containing as the copper corrosion inhibitor a 1,3,4-thiadiazole polysulfide characterized by the formula: ##EQU2## where R.sup.1 is a hydrocarbon radical such as alkyl of from 1 to 30 carbons and x is an integer of from 0 to 8. It is interesting to note that the fact that this particular sulfurous material is an effective copper corrosion inhibitor whereas the aforementioned sulfur containing anti-wear and extreme pressure additives are corrosive to copper. This indicates unpredictability regarding the function of sulfur compounds as copper corrosion inhibiting agents.